Simple WP function to insert admin notices and errors.

admin_notice.php

/**
 * Include
 *     add_action('admin_notices', 'lm_admin_message');
 * in your setup, then call
 *     lm_admin_message('error', 'the message, fool!'); or
 *     lm_admin_message('notice', 'the notice, fool!');
 * to display the admin message.
 *
 * @param string|null $type Supports "error" and "notice" (or null if it's the setup hook call)
 * @param string|null $message Body of your message (or null if it's the setup hook call)
 */
function lm_admin_message($type = null, $message = null) {
	if(!is_admin())
		return;

	if(empty($type) && empty($message)) {
		if(!function_exists('display')) {
			function display($type, $messages) {
				if(!empty($messages) && is_array($messages)) {
					$o = '<div class="' . $type . '">';

					foreach($messages as $message)
						$o .= '<p>' . $message . '</p>';

					echo $o . '</div>';
				}
			}
		}

		if(!empty($_GET['lm_errors'])) {
			$errors = get_option('lm_errors');
			display('error', $errors);
			delete_option('lm_errors');
		}

		if(!empty($_GET['lm_notices'])) {
			$notices = get_option('lm_notices');
			display('updated', $notices);
			delete_option('lm_notices');
		}

		return;
	}

	switch($type) {
		case 'error':   $type = 'errors';   break;
		case 'notice':  $type = 'notices';  break;
		default:        return;             break;
	}

	$items = get_option('lm_' . $type);

	if(!$items)
		$items = array();

	update_option('lm_' . $type, array_merge($items, array( $message )));
	add_filter('redirect_post_location', 'redirect_post_location_' . $type);

	if(!function_exists('redirect_post_location_errors')) {
		function redirect_post_location_errors($loc) {
			return add_query_arg('lm_errors', 1, remove_query_arg('message', $loc));
		}
	}

	if(!function_exists('redirect_post_location_notices')) {
		function redirect_post_location_notices($loc) {
			return add_query_arg('lm_notices', 1, remove_query_arg('message', $loc));
		}
	}
}

Might also want to mark the post as a draft, if you used this to show a user errors in their input.